MT-053: Op Amp Distortion: HD, THD, THD + N, …

1 , 10/08, WK Page 1 of 8 MT-053 TUTORIAL Op Amp distortion : HD, THD, THD + N, IMD, SFDR, MTPR HARMONIC distortion (HD) AND TOTAL HARMONIC distortion (THD) The dynamic range of an op amp may be defined in several ways. One of the most common ways is to specify harmonic distortion , total harmonic distortion (THD), or total harmonic distortion plus noise (THD + N). Other related specifications include intermodulation distortion (IMD), intercept points (IP2, IP3), spurious free dynamic range (SFDR), and multitone power ratio (MTPR).

2 Harmonic distortion is simply the ratio of the rms value of the harmonic of interest (2nd, 3rd, etc.) to the rms signal level. In audio applications it is usually expressed as a percentage, but in communications applications it is more often expressed in dB. It is measured by applying a spectrally pure sinewave to an amplifier and observing the output of the amplifier with a spectrum analyzer. Total Harmonic distortion (THD) is the ratio of the root-sum-square value of all the harmonics (2 , 3 , 4 , etc.)

3 To the rms signal level. Generally speaking, only the first five or six harmonics are significant in the THD measurement. In many practical situations, there is negligible error if only the second and third harmonics are included, since the higher order terms most often are greatly reduced in amplitude. TOTAL HARMONIC distortion PLUS NOISE (THD + N) Total Harmonic distortion Plus Noise (THD + N) is the ratio of the root-sum-square of all the harmonics and noise components over a specified bandwidth to the rms signal level.

4 It is important to note that the THD measurement does not include noise terms, while THD + N does. The noise term in the THD + N measurement must be integrated over the measurement bandwidth, and this bandwidth must be specified in order for the measurement to be meaningful. In narrow-band applications, the level of the noise may be reduced by filtering, in turn lowering the THD + N which increases the signal-to-noise ratio (SNR). Many times (especially in audio applications) when a THD specification is quoted, the manufacturer really means THD + N, since most measurement systems do not differentiate harmonically related signals from the other signals.

5 The THD + N measurement is generally made by notching out the fundamental signal (to prevent overdrive) and measuring the residual signal which includes both noise and distortion components. Special analyzers made by Audio Precision are popular in audio applications for making THD + N measurements. The definitions of THD and THD + N are summarized in Figure 1. MT-053 Vs= Signal Amplitude (RMS Volts)V2= Second Harmonic Amplitude (RMS Volts)Vn= nth Harmonic Amplitude (RMS Volts)Vnoise= RMS value of noise over measurement bandwidthTHD + N =THD = Vs VsV22+ V32+ V42+.

6 + Vn2+ Vnoise2V22+ V32+ V42+ .. + Vn2 Figure 1: THD and THD + N Definitions INTERMODULATION distortion (IMD) When a spectrally pure sinewave passes through an amplifier (or other active device), various harmonic distortion products are produced depending upon the nature and the severity of the non-linearity. However, simply measuring harmonic distortion produced by single tone sinewaves of various frequencies does not give all the information required to evaluate the amplifier's potential performance in a communications application.

7 In most communications systems there are a number of channels which are "stacked" in frequency. It is often required that an amplifier be rated in terms of the intermodulation distortion (IMD) produced with two or more specified tones applied. Intermodulation distortion products are of special interest in the IF and RF area, and a major concern in the design of radio receivers. Rather than simply examining the harmonic distortion or total harmonic distortion (THD) produced by a single tone sinewave input, it is often required to look at the distortion products produced by two tones.

8 As shown in Figure 2, two tones will produce second and third order intermodulation products. The example shows the second and third order products produced by applying two frequencies, f1 and f2, to a nonlinear device. The second order products located at f2 + f1 and f2 f1 are located far away from the two tones, and may be removed by filtering. The third order products located at 2f1 + f2 and 2f2 + f1 may likewise be filtered. The third order products located at 2f1 f2 and 2f2 f1, however, are close to the original tones, and filtering them is difficult.

9 Third order IMD products are especially troublesome in multi-channel communications systems where the channel separation is constant across the frequency band. Third-order IMD products from large signals (blockers) can mask out smaller signals. Page 2 of 8 MT-053 FREQUENCY: MHz2 = SECOND ORDER IMD PRODUCTS3 = THIRD ORDER IMD PRODUCTSNOTE: f1= 5 MHz, f2= 6 MHzf2 -f12f1 -f22f2 -f1f1f22f12f2f2 + f12f1 + f23f12f2 + f13f22332331 4 5 6 7 10 11 12 15 16 17 18 Figure 2.

10 Second and Third Order Intermodulation distortion Products INTERCEPT POINTS AND 1 dB COMPRESSION POINT Third order IMD is often specified in terms of the third order intercept point, as is shown by Figure 3, below. Two spectrally pure tones are applied to the system. The output signal power in a single tone (in dBm) as well as the relative amplitude of the third-order products (referenced to a single tone) is plotted as a function of input signal power. The fundamental is shown by the slope = 1 curve in the diagram.